There is a need for low bend loss optical fibers, particularly for optical fibers utilized in so-called “access” and fiber to the premises (FTTx) optical networks. Low bend loss optical fiber can be deployed in such networks in a manner which reduces bend losses in optical signals transmitted through the optical fiber. Some applications or optical components that utilize optical fiber can impose physical demands, such as tight bend radii, compression of optical fiber, etc., that induce bend losses. These fibers include a fluorine doped region (i.e., trench) in the cladding to minimize bend losses.
More specifically, bend insensitive optical fibers usually have an inner core region, surrounded by an inner cladding region, followed by a trench (low refractive index region, also referred as a trench herein) and an outer cladding region. The inner and outer cladding regions are typically pure silica, with the trench being silica doped with fluorine to achieve the negative index of refraction relative to silica. The optical fibers that have a trench shape such that the inside of the trench has refractive index that is significantly less negative than that on the outside of the trench (i.e. trapezoidal or triangular trench profiles are bend insensitive and have similar bend performance as compared to fibers with rectangular trenches, but also have larger mode field diameters (MFDs). Optical fiber profiles with these refractive index profiles can be achieved using a non-uniform fluorine doping process, where the core, inner cladding and the trench are deposited as a soot preform in a single step, and the fluorine doping is performed from outside at temperatures above 1300° C. during the sintering of the soot preform. However, Applicants discovered that when these fluorine doped preforms are sintered, the sintering process often results in preform failure.
Thus, a need exists for alternative methods to produce preforms having trench profiles where the inside of the trench has a much lower fluorine concentration compared to the outside of the trench, and which is not likely to result in preform failure due to fast sinter rates at high temperatures.